1. Field of Invention
This invention relates to a novel composition for a charge transport layer of a photoreceptor used in electrophotographic devices such as photocopiers. More in particular, the invention relates to a particular composition for a charge transport layer that includes binder and one or more charge transporting molecules in specified amounts, along with optional anti-oxidants and acid dopants.
2. Description of Related Art
In the art of electrophotography, an electrophotographic imaging member or plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation, for example light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photosensitive members.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and optional protective or overcoating layer(s). The imaging members can take several forms, including flexible belts, rigid drums, etc. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness.
Typical electrophotographic imaging members (for example, photoreceptors) comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated, for example, in U.S. Pat. No. 4,265,990, which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photogenerating layer is sandwiched between the contiguous charge transport layer and the supporting conductive layer, and the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge.
As more advanced, complex, highly sophisticated, electrophotographic copiers, duplicators and printers are developed, greater demands are placed on the photoreceptor to meet stringent requirements for the production of high quality images.
One type of multi-layered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer, a charge generating layer, and a charge transport layer. The charge transport layer often comprises an activating small molecule dispersed or dissolved in a polymeric film forming binder. Generally, the polymeric film forming binder in the transport layer is electrically inactive by itself and becomes electrically active when it contains the activating molecule. The expression “electrically active” means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer in order to discharge a surface charge on the active layer. The multi-layered type of photoreceptor may also comprise additional layers such as an anti-curl backing layer, required when layers possess different coefficient of thermal expansion values, an adhesive layer, and an overcoating layer. Commercial high quality photoreceptors have been produced which utilize an anti-curl coating.
Photoreceptors have been developed which comprise charge transfer complexes prepared with polymeric molecules. For example, charge transport complexes formed with polyvinyl carbazole are disclosed in U.S. Pat. Nos. 4,047,948, 4,346,158 and 4,388,392. Photoreceptors utilizing polyvinyl carbazole layers, as compared with current photoreceptor requirements, exhibit relatively poor xerographic performance in both electrical and mechanical properties. Polymeric arylamine molecules prepared from the condensation of di-secondary amine with a di-iodo aryl compound are disclosed in European Patent Publication No. 34,425, published Aug. 26, 1981. Since these polymers are extremely brittle and form films which are very susceptible to physical damage, their use in a flexible belt configuration is precluded.
Photoreceptors having charge transport layers containing charge transporting arylamine polymers have been described in the patent literature, for example in U.S. Pat. Nos. 4,806,443, 4,806,443, 4,801,517, 4,818,650, 4,959,288, 5,202,408 and 5,262,512, the entire disclosures of these patents being incorporated herein by reference. These polymers tend to possess poor mechanical properties and are soft and non-robust.
Other photoreceptors having charge transport layers containing a charge transport molecule and a binder mixture comprising a polycarbonate and an elastomeric block copolymer have been described in U.S. Pat. No. 5,122,429.
U.S. Pat. No. 6,645,686 describes an electrophotographic imaging member having a charge transport layer that is comprised of a binder and charge transport molecules, wherein the binder eliminates or minimizes crystallization of the charge transport molecules. Specific binders are polycarbonate binders such as PCZ-800, PCZ-500, and PCZ-400 polycarbonate resin.
U.S. Pat. No. 6,194,111 describes a crosslinkable charge transport layer material for a photoconductor that includes at least one poly(arylene ether alcohol), at least one polyisocyanate crosslinking agent and at least one charge transport material dispersed in a solvent. The crosslinkable charge transport layer material is crosslinked following application of the coating solution to the photoconductor. The photoconductor including such crosslinked charge transport layer exhibits excellent wear resistance so as to have long life, thereby reducing the cost of electrophotographic printing machines employing such photoconductors therein.
One of the most noticeable problems still present in current organic photoreceptors is lateral charge migration (LCM). It appears that a primary cause of LCM is an externally induced conductivity of the photoreceptor surface, which results in charge spreading of the latent electrostatic image, which image in turn is subsequently developed less precisely by toner.
There continues to be a need for improved electrophotographic imaging members, particularly imaging members that are able to achieve high quality images and exhibit no lateral charge migration on a xerographic time scale.